Transparent colored films are employed in a number of end-use applications, for example, as window films, light filters and the like. When employed as window films in the building and automotive industries, the colored transparent window films are typically adhered to glass window surfaces via a suitable adhesive so as to reduce the amount of near infrared, ultra-violet and/or visible radiation entering the building or automotive interior space. Such solar films therefore assist the occupants by providing less glare, reducing interior heating effects and the like.
Colored transparent films are typically provided by dyeing a suitable thermoplastic (preferably polyester) film substrate as is disclosed more completely in U.S. Pat. Nos. 3,989,453; 3,943,105; 3,932,126; 4,050,892 and 4,047,889 (the entire content of each being incorporated expressly hereinto by reference). Thus, the thermoplastic film substrate may be preconditioned to enhance affinity to solvent or disperse dyes, followed by contacting the preconditioned film substrate with an organic dye-containing paste or solution.
Gray-toned (i.e., so-called “smoke-colored”) transparent films are especially desirable in the market and are produced by suitable dyeing of the film substrates using the necessary proportions of red, blue and yellow organic dyes. One problem which these conventional gray-toned transparent films experience, however, is that the yellow dye component is more susceptible to light degradation as compared to the red and blue dye components. Thus, over prolonged exposure to light, the yellow dye component of the gray-toned dyed film tends to decompose (fade) thereby changing the visual appearance of the film undesirably to a more purple color tone due to the then more dominant presence of the red and blue dyes remaining in the dyed film.
It would therefore be especially desirable if colored (dyed) transparent thermoplastic films could be rendered more light stable thereby minimizing (or preventing entirely) fading and/or changing color tones over time. It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in composite film structures exhibiting a predetermined finished color tone comprised of a transparent film layer which exhibits a color deficiency as compared to the finished color tone, and a pigment which is visually associated with, and satisfies the color deficiency of, the film layer. Most preferably, the pigment is provided as a homogenous dispersion in a transparent color-matching layer positioned adjacent to the film layer. Thus, when the film and color-matching layers are viewed collectively as a unit, the perceived color tone will be that of the finished predetermined color tone. In other words, the color-matching layer provides visually an additive effect on the perceived color of the composite film structure.
As a practical matter, therefore, the color deficiency of the film layer can be selected to be that particular dye which is more light unstable and thereby more likely to degrade over time when exposed to light. The pigment in the color-matching layer (which would inherently be more color stable as compared to the dye) may then be selected to satisfy the color deficiency in the film layer. As a result, a more light stable transparent color film composite structure ensues (i.e., due to the lesser amount (if any) of more light unstable dye(s) in the film and the greater amount of more light stable pigments in the adjacent color-matching layer).
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.